Safety standards for modern automotive vehicles are becoming increasingly strict. The result of heightened standards and innovations by designers of vehicle resistant systems is twofold: 1) occupants of vehicles are safer and less likely to suffer serious injury in a collision, and 2) manufacturers are held to higher standards with little opportunity to offset costs. As a result of these trends, innovations that increase safety while retaining efficiency and low production costs are essential to manufacturers of related technology.
Airbag systems typically are composed of a restraint control module and a reaction canister. The reaction canister contains a folded airbag and an inflator with an igniter (squib). The inflator is connected to the restraint control module via conductive wires and associated connectors. The total electrical path of these wires and connectors to and from the inflator is termed the “squib loop.” Airbag deployment is initiated when vehicle acceleration sensors measure a threshold sufficient to warrant deployment. Upon this event, a signal is sent to the restraint control module. The restraint control module then provides sufficient energy to the inflator through the squib loop to initiate the discharge of inflator gas to inflate and deploy the airbag.
The restraint control module controls the overall operation of the airbag system and essentially could be viewed as the main control unit for the airbag system. As with any system based on electronic components and sensors, airbag systems and their associated electronic components, require power from a power supply in order to function properly. Specifically, in airbag safety systems, customer demand and safety standards require that the integrity of operation be ensured through diagnostic verification of the airbag ignition system. A common method for diagnostic testing is completed by testing the squib loop for proper electrical resistance throughout vehicle operation.
Historically, airbag safety restraint systems typically employed a single inflator device to release inflation gas for inflating a vehicle occupant restraint airbag in the event of a collision. For these systems, the squib loop resistance is typically composed of linear resistive components including wire, connectors, clock-springs, EMI inductors, and the squib. Due to the constant resistance of these components they can be simplified by combining the individual element resistances into a total equivalent resistance. Since all of the components of a single inflator squib loop include only linear resistance components, the typical method employed for measuring squib loop resistance involves measurement of the differential squib loop voltage while providing specific bias current(s) and determining the squib loop resistance as a function of the voltage differential and the bias current. Such measurement is done at a current level significantly below the threshold to cause ignition of the squib. While this and other similar methods are effective for squib loops with only linear resistance, this analysis is inadequate to accurately determine the performance of more advanced systems.
In response to increasingly complex performance specifications, inflatable safety restraint technology has led to the development of what has been termed “adaptive” or “smart” inflator devices and corresponding inflatable restraint systems. Some examples of the present state of the art “smart” systems employ two stages (dual stage systems) for inflators that typically utilize two separate initiator assemblies. Common implementations of these systems utilize separate dedicated wires (firing lines) to conduct each safety device function signal. The signals are sent from a restraint control module to each airbag initiator being commanded to activate. Thus, the evolution of the technology to “smart” systems has led to an increase in the number of individual squib loops, connectors, output pins, and restraint control module connectors required for providing airbag activation. As a result, such dual stage systems are typically larger in packaging, size, heavier, and more complex in operation than their single stage counterparts.
In an effort to minimize the complexity in wiring and reduce wiring and connection cost, the dual stage systems have combined multiple squib loops into a single wiring squib loop path. This design approach is lighter and more efficient in design, but has also introduced some non-linear electrical components in the squib loop to provide for isolation of the individual squibs. In the single wiring squib loop design, the wire, connectors, clock-springs, EMI inductors, and squib are all comprised of resistively linear electrical components. Additional, non-linear components, added to isolate individual squibs, can be diodes (standard p-n junction diodes, Schottky diodes, etc.) bipolar junction transistors (BJT), insulated gate bipolar transistors (IGBT), MOSFETs, or other non-linear components. Unlike the common single stage designs, the linear and non-linear components cannot be combined into a simple, linear equivalent resistance. Other forms of advanced resistant systems implement multiple activated systems. For example, in addition to an airbag initiator squib, another system feature may be an airbag tether release, airbag vent, pretensioner or other system. Accordingly, dual stage resistant systems are an example of a duplex restraint system, on providing multiple commands which may be controlled on a single pair of conductors as described above.
The added complexity of duplex systems has rendered prior practices for squib loop resistance measurement insufficient. As stated previously, diagnostic testing is completed by testing the squib loop for proper resistance, but in non-linear systems, the resistance changes at varying bias currents. The result of the addition of non-linear components is a diminished the capability to complete accurate diagnostic testing and a limited capacity to provide the safety feedback sought after by vehicle manufacturers.